GNU Linux-libre 6.1.86-gnu

[releases.git] / fs / iomap / buffered-io.c

// SPDX-License-Identifier: GPL-2.0
/*
 * Copyright (C) 2010 Red Hat, Inc.
 * Copyright (C) 2016-2019 Christoph Hellwig.
 */
#include <linux/module.h>
#include <linux/compiler.h>
#include <linux/fs.h>
#include <linux/iomap.h>
#include <linux/pagemap.h>
#include <linux/uio.h>
#include <linux/buffer_head.h>
#include <linux/dax.h>
#include <linux/writeback.h>
#include <linux/list_sort.h>
#include <linux/swap.h>
#include <linux/bio.h>
#include <linux/sched/signal.h>
#include <linux/migrate.h>
#include "trace.h"

#include "../internal.h"

#define IOEND_BATCH_SIZE        4096

/*
 * Structure allocated for each folio when block size < folio size
 * to track sub-folio uptodate status and I/O completions.
 */
struct iomap_page {
        atomic_t                read_bytes_pending;
        atomic_t                write_bytes_pending;
        spinlock_t              uptodate_lock;
        unsigned long           uptodate[];
};

static inline struct iomap_page *to_iomap_page(struct folio *folio)
{
        if (folio_test_private(folio))
                return folio_get_private(folio);
        return NULL;
}

static struct bio_set iomap_ioend_bioset;

static struct iomap_page *
iomap_page_create(struct inode *inode, struct folio *folio, unsigned int flags)
{
        struct iomap_page *iop = to_iomap_page(folio);
        unsigned int nr_blocks = i_blocks_per_folio(inode, folio);
        gfp_t gfp;

        if (iop || nr_blocks <= 1)
                return iop;

        if (flags & IOMAP_NOWAIT)
                gfp = GFP_NOWAIT;
        else
                gfp = GFP_NOFS | __GFP_NOFAIL;

        iop = kzalloc(struct_size(iop, uptodate, BITS_TO_LONGS(nr_blocks)),
                      gfp);
        if (iop) {
                spin_lock_init(&iop->uptodate_lock);
                if (folio_test_uptodate(folio))
                        bitmap_fill(iop->uptodate, nr_blocks);
                folio_attach_private(folio, iop);
        }
        return iop;
}

static void iomap_page_release(struct folio *folio)
{
        struct iomap_page *iop = folio_detach_private(folio);
        struct inode *inode = folio->mapping->host;
        unsigned int nr_blocks = i_blocks_per_folio(inode, folio);

        if (!iop)
                return;
        WARN_ON_ONCE(atomic_read(&iop->read_bytes_pending));
        WARN_ON_ONCE(atomic_read(&iop->write_bytes_pending));
        WARN_ON_ONCE(bitmap_full(iop->uptodate, nr_blocks) !=
                        folio_test_uptodate(folio));
        kfree(iop);
}

/*
 * Calculate the range inside the folio that we actually need to read.
 */
static void iomap_adjust_read_range(struct inode *inode, struct folio *folio,
                loff_t *pos, loff_t length, size_t *offp, size_t *lenp)
{
        struct iomap_page *iop = to_iomap_page(folio);
        loff_t orig_pos = *pos;
        loff_t isize = i_size_read(inode);
        unsigned block_bits = inode->i_blkbits;
        unsigned block_size = (1 << block_bits);
        size_t poff = offset_in_folio(folio, *pos);
        size_t plen = min_t(loff_t, folio_size(folio) - poff, length);
        unsigned first = poff >> block_bits;
        unsigned last = (poff + plen - 1) >> block_bits;

        /*
         * If the block size is smaller than the page size, we need to check the
         * per-block uptodate status and adjust the offset and length if needed
         * to avoid reading in already uptodate ranges.
         */
        if (iop) {
                unsigned int i;

                /* move forward for each leading block marked uptodate */
                for (i = first; i <= last; i++) {
                        if (!test_bit(i, iop->uptodate))
                                break;
                        *pos += block_size;
                        poff += block_size;
                        plen -= block_size;
                        first++;
                }

                /* truncate len if we find any trailing uptodate block(s) */
                for ( ; i <= last; i++) {
                        if (test_bit(i, iop->uptodate)) {
                                plen -= (last - i + 1) * block_size;
                                last = i - 1;
                                break;
                        }
                }
        }

        /*
         * If the extent spans the block that contains the i_size, we need to
         * handle both halves separately so that we properly zero data in the
         * page cache for blocks that are entirely outside of i_size.
         */
        if (orig_pos <= isize && orig_pos + length > isize) {
                unsigned end = offset_in_folio(folio, isize - 1) >> block_bits;

                if (first <= end && last > end)
                        plen -= (last - end) * block_size;
        }

        *offp = poff;
        *lenp = plen;
}

static void iomap_iop_set_range_uptodate(struct folio *folio,
                struct iomap_page *iop, size_t off, size_t len)
{
        struct inode *inode = folio->mapping->host;
        unsigned first = off >> inode->i_blkbits;
        unsigned last = (off + len - 1) >> inode->i_blkbits;
        unsigned long flags;

        spin_lock_irqsave(&iop->uptodate_lock, flags);
        bitmap_set(iop->uptodate, first, last - first + 1);
        if (bitmap_full(iop->uptodate, i_blocks_per_folio(inode, folio)))
                folio_mark_uptodate(folio);
        spin_unlock_irqrestore(&iop->uptodate_lock, flags);
}

static void iomap_set_range_uptodate(struct folio *folio,
                struct iomap_page *iop, size_t off, size_t len)
{
        if (iop)
                iomap_iop_set_range_uptodate(folio, iop, off, len);
        else
                folio_mark_uptodate(folio);
}

static void iomap_finish_folio_read(struct folio *folio, size_t offset,
                size_t len, int error)
{
        struct iomap_page *iop = to_iomap_page(folio);

        if (unlikely(error)) {
                folio_clear_uptodate(folio);
                folio_set_error(folio);
        } else {
                iomap_set_range_uptodate(folio, iop, offset, len);
        }

        if (!iop || atomic_sub_and_test(len, &iop->read_bytes_pending))
                folio_unlock(folio);
}

static void iomap_read_end_io(struct bio *bio)
{
        int error = blk_status_to_errno(bio->bi_status);
        struct folio_iter fi;

        bio_for_each_folio_all(fi, bio)
                iomap_finish_folio_read(fi.folio, fi.offset, fi.length, error);
        bio_put(bio);
}

struct iomap_readpage_ctx {
        struct folio            *cur_folio;
        bool                    cur_folio_in_bio;
        struct bio              *bio;
        struct readahead_control *rac;
};

/**
 * iomap_read_inline_data - copy inline data into the page cache
 * @iter: iteration structure
 * @folio: folio to copy to
 *
 * Copy the inline data in @iter into @folio and zero out the rest of the folio.
 * Only a single IOMAP_INLINE extent is allowed at the end of each file.
 * Returns zero for success to complete the read, or the usual negative errno.
 */
static int iomap_read_inline_data(const struct iomap_iter *iter,
                struct folio *folio)
{
        struct iomap_page *iop;
        const struct iomap *iomap = iomap_iter_srcmap(iter);
        size_t size = i_size_read(iter->inode) - iomap->offset;
        size_t poff = offset_in_page(iomap->offset);
        size_t offset = offset_in_folio(folio, iomap->offset);
        void *addr;

        if (folio_test_uptodate(folio))
                return 0;

        if (WARN_ON_ONCE(size > PAGE_SIZE - poff))
                return -EIO;
        if (WARN_ON_ONCE(size > PAGE_SIZE -
                         offset_in_page(iomap->inline_data)))
                return -EIO;
        if (WARN_ON_ONCE(size > iomap->length))
                return -EIO;
        if (offset > 0)
                iop = iomap_page_create(iter->inode, folio, iter->flags);
        else
                iop = to_iomap_page(folio);

        addr = kmap_local_folio(folio, offset);
        memcpy(addr, iomap->inline_data, size);
        memset(addr + size, 0, PAGE_SIZE - poff - size);
        kunmap_local(addr);
        iomap_set_range_uptodate(folio, iop, offset, PAGE_SIZE - poff);
        return 0;
}

static inline bool iomap_block_needs_zeroing(const struct iomap_iter *iter,
                loff_t pos)
{
        const struct iomap *srcmap = iomap_iter_srcmap(iter);

        return srcmap->type != IOMAP_MAPPED ||
                (srcmap->flags & IOMAP_F_NEW) ||
                pos >= i_size_read(iter->inode);
}

static loff_t iomap_readpage_iter(const struct iomap_iter *iter,
                struct iomap_readpage_ctx *ctx, loff_t offset)
{
        const struct iomap *iomap = &iter->iomap;
        loff_t pos = iter->pos + offset;
        loff_t length = iomap_length(iter) - offset;
        struct folio *folio = ctx->cur_folio;
        struct iomap_page *iop;
        loff_t orig_pos = pos;
        size_t poff, plen;
        sector_t sector;

        if (iomap->type == IOMAP_INLINE)
                return iomap_read_inline_data(iter, folio);

        /* zero post-eof blocks as the page may be mapped */
        iop = iomap_page_create(iter->inode, folio, iter->flags);
        iomap_adjust_read_range(iter->inode, folio, &pos, length, &poff, &plen);
        if (plen == 0)
                goto done;

        if (iomap_block_needs_zeroing(iter, pos)) {
                folio_zero_range(folio, poff, plen);
                iomap_set_range_uptodate(folio, iop, poff, plen);
                goto done;
        }

        ctx->cur_folio_in_bio = true;
        if (iop)
                atomic_add(plen, &iop->read_bytes_pending);

        sector = iomap_sector(iomap, pos);
        if (!ctx->bio ||
            bio_end_sector(ctx->bio) != sector ||
            !bio_add_folio(ctx->bio, folio, plen, poff)) {
                gfp_t gfp = mapping_gfp_constraint(folio->mapping, GFP_KERNEL);
                gfp_t orig_gfp = gfp;
                unsigned int nr_vecs = DIV_ROUND_UP(length, PAGE_SIZE);

                if (ctx->bio)
                        submit_bio(ctx->bio);

                if (ctx->rac) /* same as readahead_gfp_mask */
                        gfp |= __GFP_NORETRY | __GFP_NOWARN;
                ctx->bio = bio_alloc(iomap->bdev, bio_max_segs(nr_vecs),
                                     REQ_OP_READ, gfp);
                /*
                 * If the bio_alloc fails, try it again for a single page to
                 * avoid having to deal with partial page reads.  This emulates
                 * what do_mpage_read_folio does.
                 */
                if (!ctx->bio) {
                        ctx->bio = bio_alloc(iomap->bdev, 1, REQ_OP_READ,
                                             orig_gfp);
                }
                if (ctx->rac)
                        ctx->bio->bi_opf |= REQ_RAHEAD;
                ctx->bio->bi_iter.bi_sector = sector;
                ctx->bio->bi_end_io = iomap_read_end_io;
                bio_add_folio(ctx->bio, folio, plen, poff);
        }

done:
        /*
         * Move the caller beyond our range so that it keeps making progress.
         * For that, we have to include any leading non-uptodate ranges, but
         * we can skip trailing ones as they will be handled in the next
         * iteration.
         */
        return pos - orig_pos + plen;
}

int iomap_read_folio(struct folio *folio, const struct iomap_ops *ops)
{
        struct iomap_iter iter = {
                .inode          = folio->mapping->host,
                .pos            = folio_pos(folio),
                .len            = folio_size(folio),
        };
        struct iomap_readpage_ctx ctx = {
                .cur_folio      = folio,
        };
        int ret;

        trace_iomap_readpage(iter.inode, 1);

        while ((ret = iomap_iter(&iter, ops)) > 0)
                iter.processed = iomap_readpage_iter(&iter, &ctx, 0);

        if (ret < 0)
                folio_set_error(folio);

        if (ctx.bio) {
                submit_bio(ctx.bio);
                WARN_ON_ONCE(!ctx.cur_folio_in_bio);
        } else {
                WARN_ON_ONCE(ctx.cur_folio_in_bio);
                folio_unlock(folio);
        }

        /*
         * Just like mpage_readahead and block_read_full_folio, we always
         * return 0 and just set the folio error flag on errors.  This
         * should be cleaned up throughout the stack eventually.
         */
        return 0;
}
EXPORT_SYMBOL_GPL(iomap_read_folio);

static loff_t iomap_readahead_iter(const struct iomap_iter *iter,
                struct iomap_readpage_ctx *ctx)
{
        loff_t length = iomap_length(iter);
        loff_t done, ret;

        for (done = 0; done < length; done += ret) {
                if (ctx->cur_folio &&
                    offset_in_folio(ctx->cur_folio, iter->pos + done) == 0) {
                        if (!ctx->cur_folio_in_bio)
                                folio_unlock(ctx->cur_folio);
                        ctx->cur_folio = NULL;
                }
                if (!ctx->cur_folio) {
                        ctx->cur_folio = readahead_folio(ctx->rac);
                        ctx->cur_folio_in_bio = false;
                }
                ret = iomap_readpage_iter(iter, ctx, done);
                if (ret <= 0)
                        return ret;
        }

        return done;
}

/**
 * iomap_readahead - Attempt to read pages from a file.
 * @rac: Describes the pages to be read.
 * @ops: The operations vector for the filesystem.
 *
 * This function is for filesystems to call to implement their readahead
 * address_space operation.
 *
 * Context: The @ops callbacks may submit I/O (eg to read the addresses of
 * blocks from disc), and may wait for it.  The caller may be trying to
 * access a different page, and so sleeping excessively should be avoided.
 * It may allocate memory, but should avoid costly allocations.  This
 * function is called with memalloc_nofs set, so allocations will not cause
 * the filesystem to be reentered.
 */
void iomap_readahead(struct readahead_control *rac, const struct iomap_ops *ops)
{
        struct iomap_iter iter = {
                .inode  = rac->mapping->host,
                .pos    = readahead_pos(rac),
                .len    = readahead_length(rac),
        };
        struct iomap_readpage_ctx ctx = {
                .rac    = rac,
        };

        trace_iomap_readahead(rac->mapping->host, readahead_count(rac));

        while (iomap_iter(&iter, ops) > 0)
                iter.processed = iomap_readahead_iter(&iter, &ctx);

        if (ctx.bio)
                submit_bio(ctx.bio);
        if (ctx.cur_folio) {
                if (!ctx.cur_folio_in_bio)
                        folio_unlock(ctx.cur_folio);
        }
}
EXPORT_SYMBOL_GPL(iomap_readahead);

/*
 * iomap_is_partially_uptodate checks whether blocks within a folio are
 * uptodate or not.
 *
 * Returns true if all blocks which correspond to the specified part
 * of the folio are uptodate.
 */
bool iomap_is_partially_uptodate(struct folio *folio, size_t from, size_t count)
{
        struct iomap_page *iop = to_iomap_page(folio);
        struct inode *inode = folio->mapping->host;
        unsigned first, last, i;

        if (!iop)
                return false;

        /* Caller's range may extend past the end of this folio */
        count = min(folio_size(folio) - from, count);

        /* First and last blocks in range within folio */
        first = from >> inode->i_blkbits;
        last = (from + count - 1) >> inode->i_blkbits;

        for (i = first; i <= last; i++)
                if (!test_bit(i, iop->uptodate))
                        return false;
        return true;
}
EXPORT_SYMBOL_GPL(iomap_is_partially_uptodate);

bool iomap_release_folio(struct folio *folio, gfp_t gfp_flags)
{
        trace_iomap_release_folio(folio->mapping->host, folio_pos(folio),
                        folio_size(folio));

        /*
         * mm accommodates an old ext3 case where clean folios might
         * not have had the dirty bit cleared.  Thus, it can send actual
         * dirty folios to ->release_folio() via shrink_active_list();
         * skip those here.
         */
        if (folio_test_dirty(folio) || folio_test_writeback(folio))
                return false;
        iomap_page_release(folio);
        return true;
}
EXPORT_SYMBOL_GPL(iomap_release_folio);

void iomap_invalidate_folio(struct folio *folio, size_t offset, size_t len)
{
        trace_iomap_invalidate_folio(folio->mapping->host,
                                        folio_pos(folio) + offset, len);

        /*
         * If we're invalidating the entire folio, clear the dirty state
         * from it and release it to avoid unnecessary buildup of the LRU.
         */
        if (offset == 0 && len == folio_size(folio)) {
                WARN_ON_ONCE(folio_test_writeback(folio));
                folio_cancel_dirty(folio);
                iomap_page_release(folio);
        }
}
EXPORT_SYMBOL_GPL(iomap_invalidate_folio);

static void
iomap_write_failed(struct inode *inode, loff_t pos, unsigned len)
{
        loff_t i_size = i_size_read(inode);

        /*
         * Only truncate newly allocated pages beyoned EOF, even if the
         * write started inside the existing inode size.
         */
        if (pos + len > i_size)
                truncate_pagecache_range(inode, max(pos, i_size),
                                         pos + len - 1);
}

static int iomap_read_folio_sync(loff_t block_start, struct folio *folio,
                size_t poff, size_t plen, const struct iomap *iomap)
{
        struct bio_vec bvec;
        struct bio bio;

        bio_init(&bio, iomap->bdev, &bvec, 1, REQ_OP_READ);
        bio.bi_iter.bi_sector = iomap_sector(iomap, block_start);
        bio_add_folio(&bio, folio, plen, poff);
        return submit_bio_wait(&bio);
}

static int __iomap_write_begin(const struct iomap_iter *iter, loff_t pos,
                size_t len, struct folio *folio)
{
        const struct iomap *srcmap = iomap_iter_srcmap(iter);
        struct iomap_page *iop;
        loff_t block_size = i_blocksize(iter->inode);
        loff_t block_start = round_down(pos, block_size);
        loff_t block_end = round_up(pos + len, block_size);
        unsigned int nr_blocks = i_blocks_per_folio(iter->inode, folio);
        size_t from = offset_in_folio(folio, pos), to = from + len;
        size_t poff, plen;

        if (folio_test_uptodate(folio))
                return 0;
        folio_clear_error(folio);

        iop = iomap_page_create(iter->inode, folio, iter->flags);
        if ((iter->flags & IOMAP_NOWAIT) && !iop && nr_blocks > 1)
                return -EAGAIN;

        do {
                iomap_adjust_read_range(iter->inode, folio, &block_start,
                                block_end - block_start, &poff, &plen);
                if (plen == 0)
                        break;

                if (!(iter->flags & IOMAP_UNSHARE) &&
                    (from <= poff || from >= poff + plen) &&
                    (to <= poff || to >= poff + plen))
                        continue;

                if (iomap_block_needs_zeroing(iter, block_start)) {
                        if (WARN_ON_ONCE(iter->flags & IOMAP_UNSHARE))
                                return -EIO;
                        folio_zero_segments(folio, poff, from, to, poff + plen);
                } else {
                        int status;

                        if (iter->flags & IOMAP_NOWAIT)
                                return -EAGAIN;

                        status = iomap_read_folio_sync(block_start, folio,
                                        poff, plen, srcmap);
                        if (status)
                                return status;
                }
                iomap_set_range_uptodate(folio, iop, poff, plen);
        } while ((block_start += plen) < block_end);

        return 0;
}

static int iomap_write_begin_inline(const struct iomap_iter *iter,
                struct folio *folio)
{
        /* needs more work for the tailpacking case; disable for now */
        if (WARN_ON_ONCE(iomap_iter_srcmap(iter)->offset != 0))
                return -EIO;
        return iomap_read_inline_data(iter, folio);
}

static int iomap_write_begin(const struct iomap_iter *iter, loff_t pos,
                size_t len, struct folio **foliop)
{
        const struct iomap_page_ops *page_ops = iter->iomap.page_ops;
        const struct iomap *srcmap = iomap_iter_srcmap(iter);
        struct folio *folio;
        unsigned fgp = FGP_LOCK | FGP_WRITE | FGP_CREAT | FGP_STABLE | FGP_NOFS;
        int status = 0;

        if (iter->flags & IOMAP_NOWAIT)
                fgp |= FGP_NOWAIT;

        BUG_ON(pos + len > iter->iomap.offset + iter->iomap.length);
        if (srcmap != &iter->iomap)
                BUG_ON(pos + len > srcmap->offset + srcmap->length);

        if (fatal_signal_pending(current))
                return -EINTR;

        if (!mapping_large_folio_support(iter->inode->i_mapping))
                len = min_t(size_t, len, PAGE_SIZE - offset_in_page(pos));

        if (page_ops && page_ops->page_prepare) {
                status = page_ops->page_prepare(iter->inode, pos, len);
                if (status)
                        return status;
        }

        folio = __filemap_get_folio(iter->inode->i_mapping, pos >> PAGE_SHIFT,
                        fgp, mapping_gfp_mask(iter->inode->i_mapping));
        if (!folio) {
                status = (iter->flags & IOMAP_NOWAIT) ? -EAGAIN : -ENOMEM;
                goto out_no_page;
        }
        if (pos + len > folio_pos(folio) + folio_size(folio))
                len = folio_pos(folio) + folio_size(folio) - pos;

        if (srcmap->type == IOMAP_INLINE)
                status = iomap_write_begin_inline(iter, folio);
        else if (srcmap->flags & IOMAP_F_BUFFER_HEAD)
                status = __block_write_begin_int(folio, pos, len, NULL, srcmap);
        else
                status = __iomap_write_begin(iter, pos, len, folio);

        if (unlikely(status))
                goto out_unlock;

        *foliop = folio;
        return 0;

out_unlock:
        folio_unlock(folio);
        folio_put(folio);
        iomap_write_failed(iter->inode, pos, len);

out_no_page:
        if (page_ops && page_ops->page_done)
                page_ops->page_done(iter->inode, pos, 0, NULL);
        return status;
}

static size_t __iomap_write_end(struct inode *inode, loff_t pos, size_t len,
                size_t copied, struct folio *folio)
{
        struct iomap_page *iop = to_iomap_page(folio);
        flush_dcache_folio(folio);

        /*
         * The blocks that were entirely written will now be uptodate, so we
         * don't have to worry about a read_folio reading them and overwriting a
         * partial write.  However, if we've encountered a short write and only
         * partially written into a block, it will not be marked uptodate, so a
         * read_folio might come in and destroy our partial write.
         *
         * Do the simplest thing and just treat any short write to a
         * non-uptodate page as a zero-length write, and force the caller to
         * redo the whole thing.
         */
        if (unlikely(copied < len && !folio_test_uptodate(folio)))
                return 0;
        iomap_set_range_uptodate(folio, iop, offset_in_folio(folio, pos), len);
        filemap_dirty_folio(inode->i_mapping, folio);
        return copied;
}

static size_t iomap_write_end_inline(const struct iomap_iter *iter,
                struct folio *folio, loff_t pos, size_t copied)
{
        const struct iomap *iomap = &iter->iomap;
        void *addr;

        WARN_ON_ONCE(!folio_test_uptodate(folio));
        BUG_ON(!iomap_inline_data_valid(iomap));

        flush_dcache_folio(folio);
        addr = kmap_local_folio(folio, pos);
        memcpy(iomap_inline_data(iomap, pos), addr, copied);
        kunmap_local(addr);

        mark_inode_dirty(iter->inode);
        return copied;
}

/* Returns the number of bytes copied.  May be 0.  Cannot be an errno. */
static size_t iomap_write_end(struct iomap_iter *iter, loff_t pos, size_t len,
                size_t copied, struct folio *folio)
{
        const struct iomap_page_ops *page_ops = iter->iomap.page_ops;
        const struct iomap *srcmap = iomap_iter_srcmap(iter);
        loff_t old_size = iter->inode->i_size;
        size_t ret;

        if (srcmap->type == IOMAP_INLINE) {
                ret = iomap_write_end_inline(iter, folio, pos, copied);
        } else if (srcmap->flags & IOMAP_F_BUFFER_HEAD) {
                ret = block_write_end(NULL, iter->inode->i_mapping, pos, len,
                                copied, &folio->page, NULL);
        } else {
                ret = __iomap_write_end(iter->inode, pos, len, copied, folio);
        }

        /*
         * Update the in-memory inode size after copying the data into the page
         * cache.  It's up to the file system to write the updated size to disk,
         * preferably after I/O completion so that no stale data is exposed.
         */
        if (pos + ret > old_size) {
                i_size_write(iter->inode, pos + ret);
                iter->iomap.flags |= IOMAP_F_SIZE_CHANGED;
        }
        folio_unlock(folio);

        if (old_size < pos)
                pagecache_isize_extended(iter->inode, old_size, pos);
        if (page_ops && page_ops->page_done)
                page_ops->page_done(iter->inode, pos, ret, &folio->page);
        folio_put(folio);

        if (ret < len)
                iomap_write_failed(iter->inode, pos + ret, len - ret);
        return ret;
}

static loff_t iomap_write_iter(struct iomap_iter *iter, struct iov_iter *i)
{
        loff_t length = iomap_length(iter);
        loff_t pos = iter->pos;
        ssize_t written = 0;
        long status = 0;
        struct address_space *mapping = iter->inode->i_mapping;
        unsigned int bdp_flags = (iter->flags & IOMAP_NOWAIT) ? BDP_ASYNC : 0;

        do {
                struct folio *folio;
                struct page *page;
                unsigned long offset;   /* Offset into pagecache page */
                unsigned long bytes;    /* Bytes to write to page */
                size_t copied;          /* Bytes copied from user */

                offset = offset_in_page(pos);
                bytes = min_t(unsigned long, PAGE_SIZE - offset,
                                                iov_iter_count(i));
again:
                status = balance_dirty_pages_ratelimited_flags(mapping,
                                                               bdp_flags);
                if (unlikely(status))
                        break;

                if (bytes > length)
                        bytes = length;

                /*
                 * Bring in the user page that we'll copy from _first_.
                 * Otherwise there's a nasty deadlock on copying from the
                 * same page as we're writing to, without it being marked
                 * up-to-date.
                 *
                 * For async buffered writes the assumption is that the user
                 * page has already been faulted in. This can be optimized by
                 * faulting the user page.
                 */
                if (unlikely(fault_in_iov_iter_readable(i, bytes) == bytes)) {
                        status = -EFAULT;
                        break;
                }

                status = iomap_write_begin(iter, pos, bytes, &folio);
                if (unlikely(status))
                        break;

                page = folio_file_page(folio, pos >> PAGE_SHIFT);
                if (mapping_writably_mapped(mapping))
                        flush_dcache_page(page);

                copied = copy_page_from_iter_atomic(page, offset, bytes, i);

                status = iomap_write_end(iter, pos, bytes, copied, folio);

                if (unlikely(copied != status))
                        iov_iter_revert(i, copied - status);

                cond_resched();
                if (unlikely(status == 0)) {
                        /*
                         * A short copy made iomap_write_end() reject the
                         * thing entirely.  Might be memory poisoning
                         * halfway through, might be a race with munmap,
                         * might be severe memory pressure.
                         */
                        if (copied)
                                bytes = copied;
                        goto again;
                }
                pos += status;
                written += status;
                length -= status;
        } while (iov_iter_count(i) && length);

        if (status == -EAGAIN) {
                iov_iter_revert(i, written);
                return -EAGAIN;
        }
        return written ? written : status;
}

ssize_t
iomap_file_buffered_write(struct kiocb *iocb, struct iov_iter *i,
                const struct iomap_ops *ops)
{
        struct iomap_iter iter = {
                .inode          = iocb->ki_filp->f_mapping->host,
                .pos            = iocb->ki_pos,
                .len            = iov_iter_count(i),
                .flags          = IOMAP_WRITE,
        };
        int ret;

        if (iocb->ki_flags & IOCB_NOWAIT)
                iter.flags |= IOMAP_NOWAIT;

        while ((ret = iomap_iter(&iter, ops)) > 0)
                iter.processed = iomap_write_iter(&iter, i);
        if (iter.pos == iocb->ki_pos)
                return ret;
        return iter.pos - iocb->ki_pos;
}
EXPORT_SYMBOL_GPL(iomap_file_buffered_write);

static loff_t iomap_unshare_iter(struct iomap_iter *iter)
{
        struct iomap *iomap = &iter->iomap;
        const struct iomap *srcmap = iomap_iter_srcmap(iter);
        loff_t pos = iter->pos;
        loff_t length = iomap_length(iter);
        long status = 0;
        loff_t written = 0;

        /* don't bother with blocks that are not shared to start with */
        if (!(iomap->flags & IOMAP_F_SHARED))
                return length;
        /* don't bother with holes or unwritten extents */
        if (srcmap->type == IOMAP_HOLE || srcmap->type == IOMAP_UNWRITTEN)
                return length;

        do {
                unsigned long offset = offset_in_page(pos);
                unsigned long bytes = min_t(loff_t, PAGE_SIZE - offset, length);
                struct folio *folio;

                status = iomap_write_begin(iter, pos, bytes, &folio);
                if (unlikely(status))
                        return status;

                status = iomap_write_end(iter, pos, bytes, bytes, folio);
                if (WARN_ON_ONCE(status == 0))
                        return -EIO;

                cond_resched();

                pos += status;
                written += status;
                length -= status;

                balance_dirty_pages_ratelimited(iter->inode->i_mapping);
        } while (length);

        return written;
}

int
iomap_file_unshare(struct inode *inode, loff_t pos, loff_t len,
                const struct iomap_ops *ops)
{
        struct iomap_iter iter = {
                .inode          = inode,
                .pos            = pos,
                .len            = len,
                .flags          = IOMAP_WRITE | IOMAP_UNSHARE,
        };
        int ret;

        while ((ret = iomap_iter(&iter, ops)) > 0)
                iter.processed = iomap_unshare_iter(&iter);
        return ret;
}
EXPORT_SYMBOL_GPL(iomap_file_unshare);

static loff_t iomap_zero_iter(struct iomap_iter *iter, bool *did_zero)
{
        const struct iomap *srcmap = iomap_iter_srcmap(iter);
        loff_t pos = iter->pos;
        loff_t length = iomap_length(iter);
        loff_t written = 0;

        /* already zeroed?  we're done. */
        if (srcmap->type == IOMAP_HOLE || srcmap->type == IOMAP_UNWRITTEN)
                return length;

        do {
                struct folio *folio;
                int status;
                size_t offset;
                size_t bytes = min_t(u64, SIZE_MAX, length);

                status = iomap_write_begin(iter, pos, bytes, &folio);
                if (status)
                        return status;

                offset = offset_in_folio(folio, pos);
                if (bytes > folio_size(folio) - offset)
                        bytes = folio_size(folio) - offset;

                folio_zero_range(folio, offset, bytes);
                folio_mark_accessed(folio);

                bytes = iomap_write_end(iter, pos, bytes, bytes, folio);
                if (WARN_ON_ONCE(bytes == 0))
                        return -EIO;

                pos += bytes;
                length -= bytes;
                written += bytes;
        } while (length > 0);

        if (did_zero)
                *did_zero = true;
        return written;
}

int
iomap_zero_range(struct inode *inode, loff_t pos, loff_t len, bool *did_zero,
                const struct iomap_ops *ops)
{
        struct iomap_iter iter = {
                .inode          = inode,
                .pos            = pos,
                .len            = len,
                .flags          = IOMAP_ZERO,
        };
        int ret;

        while ((ret = iomap_iter(&iter, ops)) > 0)
                iter.processed = iomap_zero_iter(&iter, did_zero);
        return ret;
}
EXPORT_SYMBOL_GPL(iomap_zero_range);

int
iomap_truncate_page(struct inode *inode, loff_t pos, bool *did_zero,
                const struct iomap_ops *ops)
{
        unsigned int blocksize = i_blocksize(inode);
        unsigned int off = pos & (blocksize - 1);

        /* Block boundary? Nothing to do */
        if (!off)
                return 0;
        return iomap_zero_range(inode, pos, blocksize - off, did_zero, ops);
}
EXPORT_SYMBOL_GPL(iomap_truncate_page);

static loff_t iomap_folio_mkwrite_iter(struct iomap_iter *iter,
                struct folio *folio)
{
        loff_t length = iomap_length(iter);
        int ret;

        if (iter->iomap.flags & IOMAP_F_BUFFER_HEAD) {
                ret = __block_write_begin_int(folio, iter->pos, length, NULL,
                                              &iter->iomap);
                if (ret)
                        return ret;
                block_commit_write(&folio->page, 0, length);
        } else {
                WARN_ON_ONCE(!folio_test_uptodate(folio));
                folio_mark_dirty(folio);
        }

        return length;
}

vm_fault_t iomap_page_mkwrite(struct vm_fault *vmf, const struct iomap_ops *ops)
{
        struct iomap_iter iter = {
                .inode          = file_inode(vmf->vma->vm_file),
                .flags          = IOMAP_WRITE | IOMAP_FAULT,
        };
        struct folio *folio = page_folio(vmf->page);
        ssize_t ret;

        folio_lock(folio);
        ret = folio_mkwrite_check_truncate(folio, iter.inode);
        if (ret < 0)
                goto out_unlock;
        iter.pos = folio_pos(folio);
        iter.len = ret;
        while ((ret = iomap_iter(&iter, ops)) > 0)
                iter.processed = iomap_folio_mkwrite_iter(&iter, folio);

        if (ret < 0)
                goto out_unlock;
        folio_wait_stable(folio);
        return VM_FAULT_LOCKED;
out_unlock:
        folio_unlock(folio);
        return block_page_mkwrite_return(ret);
}
EXPORT_SYMBOL_GPL(iomap_page_mkwrite);

static void iomap_finish_folio_write(struct inode *inode, struct folio *folio,
                size_t len, int error)
{
        struct iomap_page *iop = to_iomap_page(folio);

        if (error) {
                folio_set_error(folio);
                mapping_set_error(inode->i_mapping, error);
        }

        WARN_ON_ONCE(i_blocks_per_folio(inode, folio) > 1 && !iop);
        WARN_ON_ONCE(iop && atomic_read(&iop->write_bytes_pending) <= 0);

        if (!iop || atomic_sub_and_test(len, &iop->write_bytes_pending))
                folio_end_writeback(folio);
}

/*
 * We're now finished for good with this ioend structure.  Update the page
 * state, release holds on bios, and finally free up memory.  Do not use the
 * ioend after this.
 */
static u32
iomap_finish_ioend(struct iomap_ioend *ioend, int error)
{
        struct inode *inode = ioend->io_inode;
        struct bio *bio = &ioend->io_inline_bio;
        struct bio *last = ioend->io_bio, *next;
        u64 start = bio->bi_iter.bi_sector;
        loff_t offset = ioend->io_offset;
        bool quiet = bio_flagged(bio, BIO_QUIET);
        u32 folio_count = 0;

        for (bio = &ioend->io_inline_bio; bio; bio = next) {
                struct folio_iter fi;

                /*
                 * For the last bio, bi_private points to the ioend, so we
                 * need to explicitly end the iteration here.
                 */
                if (bio == last)
                        next = NULL;
                else
                        next = bio->bi_private;

                /* walk all folios in bio, ending page IO on them */
                bio_for_each_folio_all(fi, bio) {
                        iomap_finish_folio_write(inode, fi.folio, fi.length,
                                        error);
                        folio_count++;
                }
                bio_put(bio);
        }
        /* The ioend has been freed by bio_put() */

        if (unlikely(error && !quiet)) {
                printk_ratelimited(KERN_ERR
"%s: writeback error on inode %lu, offset %lld, sector %llu",
                        inode->i_sb->s_id, inode->i_ino, offset, start);
        }
        return folio_count;
}

/*
 * Ioend completion routine for merged bios. This can only be called from task
 * contexts as merged ioends can be of unbound length. Hence we have to break up
 * the writeback completions into manageable chunks to avoid long scheduler
 * holdoffs. We aim to keep scheduler holdoffs down below 10ms so that we get
 * good batch processing throughput without creating adverse scheduler latency
 * conditions.
 */
void
iomap_finish_ioends(struct iomap_ioend *ioend, int error)
{
        struct list_head tmp;
        u32 completions;

        might_sleep();

        list_replace_init(&ioend->io_list, &tmp);
        completions = iomap_finish_ioend(ioend, error);

        while (!list_empty(&tmp)) {
                if (completions > IOEND_BATCH_SIZE * 8) {
                        cond_resched();
                        completions = 0;
                }
                ioend = list_first_entry(&tmp, struct iomap_ioend, io_list);
                list_del_init(&ioend->io_list);
                completions += iomap_finish_ioend(ioend, error);
        }
}
EXPORT_SYMBOL_GPL(iomap_finish_ioends);

/*
 * We can merge two adjacent ioends if they have the same set of work to do.
 */
static bool
iomap_ioend_can_merge(struct iomap_ioend *ioend, struct iomap_ioend *next)
{
        if (ioend->io_bio->bi_status != next->io_bio->bi_status)
                return false;
        if ((ioend->io_flags & IOMAP_F_SHARED) ^
            (next->io_flags & IOMAP_F_SHARED))
                return false;
        if ((ioend->io_type == IOMAP_UNWRITTEN) ^
            (next->io_type == IOMAP_UNWRITTEN))
                return false;
        if (ioend->io_offset + ioend->io_size != next->io_offset)
                return false;
        /*
         * Do not merge physically discontiguous ioends. The filesystem
         * completion functions will have to iterate the physical
         * discontiguities even if we merge the ioends at a logical level, so
         * we don't gain anything by merging physical discontiguities here.
         *
         * We cannot use bio->bi_iter.bi_sector here as it is modified during
         * submission so does not point to the start sector of the bio at
         * completion.
         */
        if (ioend->io_sector + (ioend->io_size >> 9) != next->io_sector)
                return false;
        return true;
}

void
iomap_ioend_try_merge(struct iomap_ioend *ioend, struct list_head *more_ioends)
{
        struct iomap_ioend *next;

        INIT_LIST_HEAD(&ioend->io_list);

        while ((next = list_first_entry_or_null(more_ioends, struct iomap_ioend,
                        io_list))) {
                if (!iomap_ioend_can_merge(ioend, next))
                        break;
                list_move_tail(&next->io_list, &ioend->io_list);
                ioend->io_size += next->io_size;
        }
}
EXPORT_SYMBOL_GPL(iomap_ioend_try_merge);

static int
iomap_ioend_compare(void *priv, const struct list_head *a,
                const struct list_head *b)
{
        struct iomap_ioend *ia = container_of(a, struct iomap_ioend, io_list);
        struct iomap_ioend *ib = container_of(b, struct iomap_ioend, io_list);

        if (ia->io_offset < ib->io_offset)
                return -1;
        if (ia->io_offset > ib->io_offset)
                return 1;
        return 0;
}

void
iomap_sort_ioends(struct list_head *ioend_list)
{
        list_sort(NULL, ioend_list, iomap_ioend_compare);
}
EXPORT_SYMBOL_GPL(iomap_sort_ioends);

static void iomap_writepage_end_bio(struct bio *bio)
{
        struct iomap_ioend *ioend = bio->bi_private;

        iomap_finish_ioend(ioend, blk_status_to_errno(bio->bi_status));
}

/*
 * Submit the final bio for an ioend.
 *
 * If @error is non-zero, it means that we have a situation where some part of
 * the submission process has failed after we've marked pages for writeback
 * and unlocked them.  In this situation, we need to fail the bio instead of
 * submitting it.  This typically only happens on a filesystem shutdown.
 */
static int
iomap_submit_ioend(struct iomap_writepage_ctx *wpc, struct iomap_ioend *ioend,
                int error)
{
        ioend->io_bio->bi_private = ioend;
        ioend->io_bio->bi_end_io = iomap_writepage_end_bio;

        if (wpc->ops->prepare_ioend)
                error = wpc->ops->prepare_ioend(ioend, error);
        if (error) {
                /*
                 * If we're failing the IO now, just mark the ioend with an
                 * error and finish it.  This will run IO completion immediately
                 * as there is only one reference to the ioend at this point in
                 * time.
                 */
                ioend->io_bio->bi_status = errno_to_blk_status(error);
                bio_endio(ioend->io_bio);
                return error;
        }

        submit_bio(ioend->io_bio);
        return 0;
}

static struct iomap_ioend *
iomap_alloc_ioend(struct inode *inode, struct iomap_writepage_ctx *wpc,
                loff_t offset, sector_t sector, struct writeback_control *wbc)
{
        struct iomap_ioend *ioend;
        struct bio *bio;

        bio = bio_alloc_bioset(wpc->iomap.bdev, BIO_MAX_VECS,
                               REQ_OP_WRITE | wbc_to_write_flags(wbc),
                               GFP_NOFS, &iomap_ioend_bioset);
        bio->bi_iter.bi_sector = sector;
        wbc_init_bio(wbc, bio);

        ioend = container_of(bio, struct iomap_ioend, io_inline_bio);
        INIT_LIST_HEAD(&ioend->io_list);
        ioend->io_type = wpc->iomap.type;
        ioend->io_flags = wpc->iomap.flags;
        ioend->io_inode = inode;
        ioend->io_size = 0;
        ioend->io_folios = 0;
        ioend->io_offset = offset;
        ioend->io_bio = bio;
        ioend->io_sector = sector;
        return ioend;
}

/*
 * Allocate a new bio, and chain the old bio to the new one.
 *
 * Note that we have to perform the chaining in this unintuitive order
 * so that the bi_private linkage is set up in the right direction for the
 * traversal in iomap_finish_ioend().
 */
static struct bio *
iomap_chain_bio(struct bio *prev)
{
        struct bio *new;

        new = bio_alloc(prev->bi_bdev, BIO_MAX_VECS, prev->bi_opf, GFP_NOFS);
        bio_clone_blkg_association(new, prev);
        new->bi_iter.bi_sector = bio_end_sector(prev);

        bio_chain(prev, new);
        bio_get(prev);          /* for iomap_finish_ioend */
        submit_bio(prev);
        return new;
}

static bool
iomap_can_add_to_ioend(struct iomap_writepage_ctx *wpc, loff_t offset,
                sector_t sector)
{
        if ((wpc->iomap.flags & IOMAP_F_SHARED) !=
            (wpc->ioend->io_flags & IOMAP_F_SHARED))
                return false;
        if (wpc->iomap.type != wpc->ioend->io_type)
                return false;
        if (offset != wpc->ioend->io_offset + wpc->ioend->io_size)
                return false;
        if (sector != bio_end_sector(wpc->ioend->io_bio))
                return false;
        /*
         * Limit ioend bio chain lengths to minimise IO completion latency. This
         * also prevents long tight loops ending page writeback on all the
         * folios in the ioend.
         */
        if (wpc->ioend->io_folios >= IOEND_BATCH_SIZE)
                return false;
        return true;
}

/*
 * Test to see if we have an existing ioend structure that we could append to
 * first; otherwise finish off the current ioend and start another.
 */
static void
iomap_add_to_ioend(struct inode *inode, loff_t pos, struct folio *folio,
                struct iomap_page *iop, struct iomap_writepage_ctx *wpc,
                struct writeback_control *wbc, struct list_head *iolist)
{
        sector_t sector = iomap_sector(&wpc->iomap, pos);
        unsigned len = i_blocksize(inode);
        size_t poff = offset_in_folio(folio, pos);

        if (!wpc->ioend || !iomap_can_add_to_ioend(wpc, pos, sector)) {
                if (wpc->ioend)
                        list_add(&wpc->ioend->io_list, iolist);
                wpc->ioend = iomap_alloc_ioend(inode, wpc, pos, sector, wbc);
        }

        if (!bio_add_folio(wpc->ioend->io_bio, folio, len, poff)) {
                wpc->ioend->io_bio = iomap_chain_bio(wpc->ioend->io_bio);
                bio_add_folio(wpc->ioend->io_bio, folio, len, poff);
        }

        if (iop)
                atomic_add(len, &iop->write_bytes_pending);
        wpc->ioend->io_size += len;
        wbc_account_cgroup_owner(wbc, &folio->page, len);
}

/*
 * We implement an immediate ioend submission policy here to avoid needing to
 * chain multiple ioends and hence nest mempool allocations which can violate
 * the forward progress guarantees we need to provide. The current ioend we're
 * adding blocks to is cached in the writepage context, and if the new block
 * doesn't append to the cached ioend, it will create a new ioend and cache that
 * instead.
 *
 * If a new ioend is created and cached, the old ioend is returned and queued
 * locally for submission once the entire page is processed or an error has been
 * detected.  While ioends are submitted immediately after they are completed,
 * batching optimisations are provided by higher level block plugging.
 *
 * At the end of a writeback pass, there will be a cached ioend remaining on the
 * writepage context that the caller will need to submit.
 */
static int
iomap_writepage_map(struct iomap_writepage_ctx *wpc,
                struct writeback_control *wbc, struct inode *inode,
                struct folio *folio, u64 end_pos)
{
        struct iomap_page *iop = iomap_page_create(inode, folio, 0);
        struct iomap_ioend *ioend, *next;
        unsigned len = i_blocksize(inode);
        unsigned nblocks = i_blocks_per_folio(inode, folio);
        u64 pos = folio_pos(folio);
        int error = 0, count = 0, i;
        LIST_HEAD(submit_list);

        WARN_ON_ONCE(iop && atomic_read(&iop->write_bytes_pending) != 0);

        /*
         * Walk through the folio to find areas to write back. If we
         * run off the end of the current map or find the current map
         * invalid, grab a new one.
         */
        for (i = 0; i < nblocks && pos < end_pos; i++, pos += len) {
                if (iop && !test_bit(i, iop->uptodate))
                        continue;

                error = wpc->ops->map_blocks(wpc, inode, pos);
                if (error)
                        break;
                trace_iomap_writepage_map(inode, &wpc->iomap);
                if (WARN_ON_ONCE(wpc->iomap.type == IOMAP_INLINE))
                        continue;
                if (wpc->iomap.type == IOMAP_HOLE)
                        continue;
                iomap_add_to_ioend(inode, pos, folio, iop, wpc, wbc,
                                 &submit_list);
                count++;
        }
        if (count)
                wpc->ioend->io_folios++;

        WARN_ON_ONCE(!wpc->ioend && !list_empty(&submit_list));
        WARN_ON_ONCE(!folio_test_locked(folio));
        WARN_ON_ONCE(folio_test_writeback(folio));
        WARN_ON_ONCE(folio_test_dirty(folio));

        /*
         * We cannot cancel the ioend directly here on error.  We may have
         * already set other pages under writeback and hence we have to run I/O
         * completion to mark the error state of the pages under writeback
         * appropriately.
         */
        if (unlikely(error)) {
                /*
                 * Let the filesystem know what portion of the current page
                 * failed to map. If the page hasn't been added to ioend, it
                 * won't be affected by I/O completion and we must unlock it
                 * now.
                 */
                if (wpc->ops->discard_folio)
                        wpc->ops->discard_folio(folio, pos);
                if (!count) {
                        folio_unlock(folio);
                        goto done;
                }
        }

        folio_start_writeback(folio);
        folio_unlock(folio);

        /*
         * Preserve the original error if there was one; catch
         * submission errors here and propagate into subsequent ioend
         * submissions.
         */
        list_for_each_entry_safe(ioend, next, &submit_list, io_list) {
                int error2;

                list_del_init(&ioend->io_list);
                error2 = iomap_submit_ioend(wpc, ioend, error);
                if (error2 && !error)
                        error = error2;
        }

        /*
         * We can end up here with no error and nothing to write only if we race
         * with a partial page truncate on a sub-page block sized filesystem.
         */
        if (!count)
                folio_end_writeback(folio);
done:
        mapping_set_error(inode->i_mapping, error);
        return error;
}

/*
 * Write out a dirty page.
 *
 * For delalloc space on the page, we need to allocate space and flush it.
 * For unwritten space on the page, we need to start the conversion to
 * regular allocated space.
 */
static int
iomap_do_writepage(struct page *page, struct writeback_control *wbc, void *data)
{
        struct folio *folio = page_folio(page);
        struct iomap_writepage_ctx *wpc = data;
        struct inode *inode = folio->mapping->host;
        u64 end_pos, isize;

        trace_iomap_writepage(inode, folio_pos(folio), folio_size(folio));

        /*
         * Refuse to write the folio out if we're called from reclaim context.
         *
         * This avoids stack overflows when called from deeply used stacks in
         * random callers for direct reclaim or memcg reclaim.  We explicitly
         * allow reclaim from kswapd as the stack usage there is relatively low.
         *
         * This should never happen except in the case of a VM regression so
         * warn about it.
         */
        if (WARN_ON_ONCE((current->flags & (PF_MEMALLOC|PF_KSWAPD)) ==
                        PF_MEMALLOC))
                goto redirty;

        /*
         * Is this folio beyond the end of the file?
         *
         * The folio index is less than the end_index, adjust the end_pos
         * to the highest offset that this folio should represent.
         * -----------------------------------------------------
         * |                    file mapping           | <EOF> |
         * -----------------------------------------------------
         * | Page ... | Page N-2 | Page N-1 |  Page N  |       |
         * ^--------------------------------^----------|--------
         * |     desired writeback range    |      see else    |
         * ---------------------------------^------------------|
         */
        isize = i_size_read(inode);
        end_pos = folio_pos(folio) + folio_size(folio);
        if (end_pos > isize) {
                /*
                 * Check whether the page to write out is beyond or straddles
                 * i_size or not.
                 * -------------------------------------------------------
                 * |            file mapping                    | <EOF>  |
                 * -------------------------------------------------------
                 * | Page ... | Page N-2 | Page N-1 |  Page N   | Beyond |
                 * ^--------------------------------^-----------|---------
                 * |                                |      Straddles     |
                 * ---------------------------------^-----------|--------|
                 */
                size_t poff = offset_in_folio(folio, isize);
                pgoff_t end_index = isize >> PAGE_SHIFT;

                /*
                 * Skip the page if it's fully outside i_size, e.g.
                 * due to a truncate operation that's in progress.  We've
                 * cleaned this page and truncate will finish things off for
                 * us.
                 *
                 * Note that the end_index is unsigned long.  If the given
                 * offset is greater than 16TB on a 32-bit system then if we
                 * checked if the page is fully outside i_size with
                 * "if (page->index >= end_index + 1)", "end_index + 1" would
                 * overflow and evaluate to 0.  Hence this page would be
                 * redirtied and written out repeatedly, which would result in
                 * an infinite loop; the user program performing this operation
                 * would hang.  Instead, we can detect this situation by
                 * checking if the page is totally beyond i_size or if its
                 * offset is just equal to the EOF.
                 */
                if (folio->index > end_index ||
                    (folio->index == end_index && poff == 0))
                        goto unlock;

                /*
                 * The page straddles i_size.  It must be zeroed out on each
                 * and every writepage invocation because it may be mmapped.
                 * "A file is mapped in multiples of the page size.  For a file
                 * that is not a multiple of the page size, the remaining
                 * memory is zeroed when mapped, and writes to that region are
                 * not written out to the file."
                 */
                folio_zero_segment(folio, poff, folio_size(folio));
                end_pos = isize;
        }

        return iomap_writepage_map(wpc, wbc, inode, folio, end_pos);

redirty:
        folio_redirty_for_writepage(wbc, folio);
unlock:
        folio_unlock(folio);
        return 0;
}

int
iomap_writepages(struct address_space *mapping, struct writeback_control *wbc,
                struct iomap_writepage_ctx *wpc,
                const struct iomap_writeback_ops *ops)
{
        int                     ret;

        wpc->ops = ops;
        ret = write_cache_pages(mapping, wbc, iomap_do_writepage, wpc);
        if (!wpc->ioend)
                return ret;
        return iomap_submit_ioend(wpc, wpc->ioend, ret);
}
EXPORT_SYMBOL_GPL(iomap_writepages);

static int __init iomap_init(void)
{
        return bioset_init(&iomap_ioend_bioset, 4 * (PAGE_SIZE / SECTOR_SIZE),
                           offsetof(struct iomap_ioend, io_inline_bio),
                           BIOSET_NEED_BVECS);
}
fs_initcall(iomap_init);